hi, I have just come across this topic, and hope you guys are still out there!
The Slipper 42 was indeed originally designed by Holman and Pye as the Rancher 41. The yard who contracted with Tylers of Tonbridge UK to mould the Rancher 41 for them went bust - cannot remember their name. So Tylers had the moulds lying in their yard. Tylers decided to adapt the moulds and added about 6 inches onto the freeboard and made up a new mould for a centre cockpit deck. I think they added a reverse transom also.
I bought hull P15 from Tylers, but was referred, by Tylers, to a bunch of crooks trading under the name of the Swanwick Yacht Agency to put in some larger water and fuel tanks as we were planning some long distance cruising. It was them who coined the name of Slipper - nothing to do with the Van de Stadt designs of the same name.
I sold my boat to an American pilot, David someone, in 1982 and lost touch with the boat. I happened upon it for sale in the San Juan Islands in 2009. She had languished first in a Fort Lauderdale yard and then one in California, but had been completely renovated by the guy selling the boat. My name is Robin Bromley-Martin, so the Robin referred to in this discussion may be me!
In case you didn't see it, this is what I currently have on the boat:http://sailboatdata.com/viewrecord.asp?class_id=4741
I may edit this to include new information you have provided.
I wish I had a better drawing and a little more detailed specs. etc.